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Appendix VL

readings are taken on a micrometer scale in the eyepiece of a
microscope to an estimate of one 5o,oooth of an inch. In mirror
extensometers a beam of light is deflected by a mirror that is
tilted by the strained specimen, the deflection being meanwhile
measured within a telescope. The apparatus to be described
can be read to an accuracy of one i,2oo,oooth of an inch,
which is an immense advance upon that of other forms.
Naturally such precision is not required in commercial instru-
ments, but is nevertheless of great value in laboratory operations.
Two important requirements must be noticed as belonging to all
extensometers of great delicacy: the measurements must be taken
on the exact centre line of the specimen, and the instrument
must not be touched by hand after being once set. Dr. John
Morrow has devised a most successful extensometer embodying
these and other principles. Referring to Fig. 952, two loose rings
B and c are held on the specimen A by the set screws E E and
D D, round which they pivot, and the pointed distance-piece K
also provides two other pivots between its ends and the rings.
A rigid bracket F is fixed on the lower ring, to carry a fixed
mirror G, and a diamond-shaped prism j constitutes a pair of
knife edges that support the tilting mirror H. The whole appa-
ratus being bound together by the springs M and L, the experiment
is arranged as in the plan view shewn below. A scale w, placed
parallel with the axis of specimen s, is illuminated by a lamp v,
and a reflected ray travels through the screen u to the extenso-
meter mirrors, strikes them both evenly, and is further reflected
to the telescope T. Considering the upper diagram again, the
extension of the specimen between E and D causes the rings to
pivot round the ends of K, thus inducing relative motion between
the ring B and the bracket F, and tilting the mirror into the
dotted position. The appearance of the scales in the telescope
will then be as shewn at N p, the tilt having caused the p reflection
to move downward, while N remains • fixed. The scale is divided
into 4oths of an inch, and a tenth of each division is easily
estimated. Also the scale being set at a distance of 80 inches
from the mirror, the magnification without telescope is 3000 : i -

so the smallest reading is — x — x  — Ğ
40    ro   3°°°


of an inch.